Differential pressure device with compensating device



1957 w. H. MAGEARL 2,780,113

DIFFERENTIAL PRESSURE DEVICE WITH COMPENSATING DEVICE Filed April 1, 1955 2 Sheets-Sheet l FIG.-I

"(D iuiam H.mc1gec1rL firmvanzor' OLL orrleg Feb. 5, 1957 MAGEARL 2,78%103 DIFFERENTIAL PRESSURE DEVICE WITH COMPENSATING DEVICE Filed April 1, 1953 2 Sheets-Sheet 2 LOW b W 941% QLLorQeS DlFFERENTIAL PRESSURE DEVICE WITH COMPENSATING DEVICE William H. Magearl, Baton Rouge, La., assignor to Esso Research and Engineering Company, a corporation of Delaware Application April 1, 1953, Serial No. 346,247

8 Claims. (Cl. 73-406) This invention relates to improvements in differential pressure transmitters or recorders and more particularly relates to a compensating means to be used when measuring differential pressures.

One specific form of the present invention is adapted for use in connection with a differential pressure indicator and provides for a compensating means to automatical- 1y compensate for the difference in height of scaled legs between an upper tap and a lower tap arranged on a tower, such as a fractionating tower. The compensating means may be used in other applications.

In the fractionation of vapors it is desirable to maintain efiicient operation of the tower with the greatest amount of throughput of vapors as is possible and consistent with good fractionation and it is also necessary to avoid flooding of the tower. One method of determining proper operation of the tower is to measure the pressure drop across the tower, that is, from top to bottom and by doing this substantially continuously a good indication will be given of the operation of the tower. Any surges or flooding of the tower will be reflected in increased pressure drops across the tower and this will be apparent by the differential pressure measurement.

Previous devices for measuring differential pressures in towers have been complicated and not entirely satisfactory.

In the preferred form of the invention compensating means are provided for differential pressure measurements across a fractionating tower. The instrument assembly includes a conventional diiferential pressure transmitter having only slight displacement of the measuring element, a seal leg compensator and a pneumatic receiver indicator or recorder. The difierential pressure measurements are taken between a top tower tap and a bottom tower tap and the compensator provides automatic compensation for the difference in the seal leg height or head of the line leading to the top tower tap and the seal leg height or head of the line leading to the bottom tower nited States Patent 2,780,103 Patented Feb. 5, 1957 '2 ovehead from the tower by line 14 and bottoms are removed through line 16. If desired a reboiler 18 may be provided at the bottom of the tower 10 for supplying heat to the bottom of the tower.

The fractionating tower contains a number of bubble trays (not shown) and there is a pressure drop from the inlet line 12 to the outlet line 14 of the tower during fractionating due to the passage of the upflowing vapors through plate openings etc. In normal operation of the tower there will be a definite pressure drop across the tower 10 from the bottom to the top and by measuring and noting this pressure drop an indication is given that the tower is operating properly. However, if surges in vapors occur or if irregularities occur in the feed rate of hydrocarbon through line 12, or if the temperature of the feed in line 12 becomes too high or too low or if the tower floods, the pressure drop through the tower 10 will change appreciably and this change in pressure drop will indicate that the tower is not operating properly.

The system for measuring pressure drop according to the present invention will now be described.

The tower 10 is provided with a top pressure tap line 22 provided with valve 24 and a bottom pressure tap Fig. 2 represents a detailed showing'of the compensat- 7 ing device applied to a conventional differential pressure indicator with parts broken away to facilitate the disclosure; and

Fig. 3 represents an enlarged longitudinal cross section of the tower 10. Fractionated vapors are taken line 26 provided with a valve 28. Seal pot 32 communicates at its top with top tap line 22 and has a line 34 provided with a valve 36. Seal leg 38 communicates with the bottom of seal pot 32 and leads to the low pressure side 39 of a conventional differential pressure device 40 such as the Foxboro differential pressure cell described in Foxboro Instrumentation Bulletin 420. Seal leg 38 adjacent pressure cell 40 is provided with a valve 42.

The Foxboro differential pressure cell measures differential pressure and operates on the force balance principle. The cell converts differential pressure into proportional air pressure and transmits this air pressure to remotely located pneumatic receiving instruments such as indicators, recorders, or controllers. An indicating g'age may be close-coupled to the differential pressure cell, connected to the output air line and calibrated to read in differential pressure measurement.

Bottom pressure tap line 26 communicates with the top of seal pot 44 which has a line 46 provided with a valve 48. Lower and shorter seal leg 50 leads from the bottom of seal pot 44, and has an extension line 52 provided with valve 54 which leads to the seal leg compensator 56 diagrammatically shown in Fig. 1 and shown in greater detail in Figs. 2 and 3. The compensator 56 communicates with the high pressure side 58 of the differential pressure cell 40.

Seal pots 32 and 44 act as reservoirs for the sealing liquid and minimize the effect of loss of any sealing liquid by evaporation over a period of time etc. Also the seal pots 32 and 44 have a larger diameter than lines 38 and 50, 52 so that variation in the level is small if there is loss of small amounts of sealing liquid. Extending between lower seal leg 50 and top seal leg 38 is line 62 provided with valves 64 and 66 and another line 68 provided with valve 72. These lines and valves are provided for putting the pressure differential cell and seal leg compensator into operation and will be described in greater detail hereinafter.

Fig. 2 shows a portion of the interior of the differential pressure cell with its central diaphragm 74 provided with a socket 76 for receiving the end of plunger 78 forming part of the seal leg compensator 56. The rest of the conventional pressure differential cell 40 is unmodified. I

Fig. 3 shows the seal leg compensator in greater detail; The housing 80 of the compensator has a reduced threaded end 82 which is threaded into the opening on the high pressure side 58 of the cell 40. The plunger 78 has a matic receiver-recorder or the like.

small en'd portion'83 leading to bevelled portion 84 spaced fromthe end leading to alarger shaft portion'86 which is slidably mounted in longitudinal opening 88 in the housing 80 of the compensator-56 and is of only slightly larger size than shaft '86. 'lhe.shaft.86 ispreferably polished to givea smooth finish to it. Grooves 92, shown dotted, are provided in the internal surface of opening 88 to permit transfer of pressure from the compensator to the high pressure sideof diaphragm 74. The housing 80 of the compensator 56 adjacent the'threaded end 82 has an enlarged shoulder portion 94 which is preferably hexagonal or other angular shape to permit use of a wrench or the like in screwing the compensator in fluid tight relation withthe opening on the high pressure side ofthe differential pressure cell 140.

Beyond the enlarged shoulder portion 94 the housing 80 of the compensator 56 has a further enlarged portion 96 which has its internal "surface cut away or bevelled as at '98 to provide an enlarged circular space 102. That is,'the housingdd ofthe'compensator '56 is hollow and circular in cross section. The inner end of shaft 86 extends through this space 192 and at its inner end is provided with an enlarged circular head 1&4. The shaft 86 and circular head 1164 are preferably integral but they may be made in separate pieces and rigidly connected together in any desired manner. The circular head 104 is provided with a groove or depression 106 for receiving one end of compensating coilspring 1418, which is arranged in the hollow portion 102 of the housing ofcompensator 56 and which is'nearly the same diameter as space 102.

The end of thehcusing 84) of compensatorfiti opposite threaded-end 32 is'open ended and internally threaded at 110 and externally threaded at 112. Screwed into internal threads 110 isthreaded adjusting head or plug 114 having a depression or cut-away'portion 116 at its inner end to receive the'other end of spring 168. The plug 114 is arranged entirely in the housing 84) of compensator 56. The plug 114 at its outer end has a hexagonal or other angular projection or depression 118 which may be engaged by a wrench or the like'to adjust the compression of the spring-108 as desired. Spring 108'holds the-end of plunger 78 in contact with socket 76 on diaphragm 74 but there is no other connection between plunger'78 and socket 76. Threaded over the exterior threads 112 of the end of the housing of the compensator 56-is aseal cap 120 to close the end of the housing and seal it. 'The cap-120 has aknurled or other exterior to assist in screwing it into sealing position.

Adjacent the enlarged portion 96thehousing '80 of compensator'56 has a pressure connection and pipe tap 122 which receives line 52 from bottom seal leg 50.

In Fig. 2 the conventional pressure differential cell is shown as having conventional vents 124 and 126. Also diagrammatically shown inFig. 2 is inlet line 128 and ontletline 130 which are conventional and which show linesfor introducing air under pressure to the differential pressure'cell and for releasing it by the cell. under pressure is used in the unmodified portion of the Foxboro cell to give an indication of the differential pressure. Also shown is 'a dial 132 which is a pressure indicator but which maybe a'recorder such as a pneu- Compensator 56 also has a vapor vent line 133.

In order to guide the movement of shaft 86 in the internal hollow space 102 adjacent enlarged portion 96 of the housing'80, a bellows (not shown) may be arranged betweenthe inner surface of enlarged portion 96 and the flat face portion 1340f the enlarged circular head 104;and surrounding and spaced from shaft 86. The spring 108 forces plunger 78 outwardly so that the outer end '83 ofplunger78 is at all times'engaged with socket 76 on'diaphra'gm 74.

'When the compensator is in operation, the se'al'legs 38:ar1'd-150, -52 are filled with a. liquid which is immiscible The air with the liquids being fractionated and which has a density'heavier than the heaviest component being fractionated in the tower. When fractionating hydrocarbons, the seal liquid is water and in the winter time in regions where water might freeze some antifreeze solution such as ethylene glycol is added to lower the freezing point of the liquid. Other anti-freeze solutions such as glycerine or othersealing liquids may be used.

To place thepressure differential instrumentation into operation the, following steps are taken. The present arrangement may be used with any size of tower and with the pressure taps as shown at the top at 22 and at the bottom at 26 and with no differential pressure across tr e diaphragm type difierential pressure transmitter which is a Foxboro differential pressure cell as above described, the output air pressure from line going to recorder 132 is adjusted to 3 lbs/sq. inch for the zero setting which is the standard accepted value for all pneumatic transmitting devicesof this type.

Then valves 72, 66; 64, '42, 54, 48, and 36 are left open. Also vent lines 124 and 126 (Fig. '2) on the cell 2d are opened. The :cell, compensator and both seal legs 38 and 5t), .52 are filled with a sealing liquid pumped through valve 72.and line '62 as from a manifold (not shown). When one side of the cell 46 and seal'leg 5t], 5'2 leading to the bottom tap 26 .and seal pct 44 are filled with the sealingliquid, valve 64 (Fig. 1) and vent 126 (Fig. 2) are closed and the pumping is continued until the other side of cell 40 and seal leg 38 leading to the top tap 22 and sea] pot 32 are "filled with the sealing liquid. Valve 66 (Fig. 1) and vent124'(Fig. 2) on cell 40 are then closed.

With valves 42, 54', '48 and 3.6 open, a differential pressure exists across the differential pressure transmitter or cell 40 equal to the difference in height or head between the two sealxlegs 3'8 and 50, 52. Then the spring 1% of the compensator56is compressed by screwing down on plug 114. Compression of spring 198 forces plunger or rod .78 against the diaphragm 74 in the cell 4G. The spring. is'compressed untilaforce equal to and opposite to the force exerted by the seal leg difference is set up thereby compensating for the difference in seal leg heights and again establishing a balance across the differential pressure 'transmiter or cell 4d. This balance, when reached, is determined by the 3 lbs./ sq. inch output air presure of the cell 40 which was established at the beginmng.

Then valve 36 in line 34 and valve 48in line 46 are closed and valve '24 in top tap line 22 and valve 23 in bottom tap line 26 are opened and'the cell 40 is then ready for operation. The differential pressure cell 4% will now read only the difference in pressure in the tower it} between-top tap'22 and bottom tap 26 as transmitted by the seal liquid in'the seal legs 'to the cell 40 and compensaeasily and readily done.

The arrangement of devices or instrument assembly above described maybe used as regular equipment on a fractionating tower to keepa continuous record of the pressure drop across the tower. By watching the recorder and the pressure drop'recorded it will be quickly determined whether or notthe tower'has been normally operating with asubstantially constant pressure drop. Any abrupt changes in pressure drop across the tower will show that there is an upset in the frac'tionating tower and this is a means. for theearly detection of tower operation upsets. By checking the pressure drop, the operation can be brought back to normal, if necessary, before any serious consequences result. By maintaining the pressure drop substantially constant, the fractionation is properly carried out and the desired product, without contamination of other products, is obtained overhead.

Or the instrument assembly above described may be mounted on a standard and be portable so that it can be moved from one tower to another to record or indicate the pressure drop through such a tower. Also the instrument assembly can be used to put a fractionating tower into operation and thereby determine the best operation for such tower by determining the best pressure drop tobe utilized to get the most efficient operation of the tower.

In such operation the conditions in the tower are changed to determine best operation. Conditions such as rate of feed, preheat of feed, reflux ratio, etc. may be varied and changes noted by the device of the present invention to arrive at the best conditions for the tower. If desired 3 taps may be used with one being intermediate the ends of the tower so that different portions of the tower may be tested or measured for performance. More than 3 taps may be used. In such testing or measuring the pressure drop across two taps is taken as above described and when 3 taps are provided, the pressure drop across the bottom and intermediate taps may be taken and/ or across the top and intermediate taps and/ or across the top and bottom taps as previously described.

The arangement of devices above described may be used with towers of any height.

The compensating element has another valuable application other than for dilferential pressure measurement across a tower. The compensating element 56 can be used on the low pressure side of a diaphragm type differential pressure transmitter or cell (not shown but similar to cell 40) and the instrument assembly used to indicate small changes in pressure when applied to a system having a relatively large working pressure. This is accomplished by compensating for a large amount of the large working pressure present, say about 85 to 95%, by means of the compensator 56 and then measuring a small amount of the upper range of the pressure with a differential pressure transmitter or cell having a small total range. With this arrangement, the low pressure side of the transmitter or cell is left open to the atmosphere and the high pressure side is connected to the system where the pressure is to be measured.

The present instrument assembly is especially adapted for use with fractionating towers in which hydrocarbon mixtures are separated into desired fractions. The compensating device has been installed on a number of fractionating towers. in one tower used for separating Cs and C4 hydrocarbons where propane goes overhead and butanes are taken off as a bottoms stream, the tower opcrates at a pressure of about 375 to 400 p. s. i. g. (pounds per square inch gage), the tower is about 79 feet high and about 8 feet in diameter. The taps 22 and 26 are about 70 feet apart. There are 30 plates in the tower. The seal liquid in the seal legs is 50-50 water and ethylene glycol. The feed stream containing primarily C3 and C4 hydrocarbons is fed into the tower through line 12 at a rate of about 14,500 barrels per day and at a temperature of about 155 F. About 4,500 barrels of C3 hydrocarbons per day are recovered through top outlet line 14.

in normal operation the pressure drop through this tower is about 3 pounds per square inch and stays about the same for efficient operation of the tower. If the pressure drop increases to 3.5 pounds per square inch in this tower, it is known that flooding of the tower occurs. If the rate of feed of hydrocarbons through line 12 is erratic or if heating of the feed hydrocarbons is irregular, this will shown up in the chart as an irregular line rather than a smooth curve. The fuel feed rate or heating of the feed is the most important of these two factors aflecting pressure drop. Abnormalities or irregularities or upsets show up immediately on the differential pressure charts and this shows the sensitivity and stability of the instrument.

The instrument assembly has also been installed on a debutanizer tower where butanes are taken overhead from gasolines or naphthas. Smooth and stable operation was indicated by the instrument assembly. However, in this tower the compensator was used in a slightly different manner in that it was installed across the lower 10 trays.

The debutanizer tower is about feet high and 9'6 in diameter and has 30 plates. It operates under a pressure of about 225 p. s. i. g. The feed to the tower comprises a naphtha fraction from which butanes and lighter are to be removed overhead. The feed is heated to about 215 F. and about 17,600 barrels per day are fed to the tower. About 11,000 barrels (as liquid) per day of butanes are recovered.

For this operation the instrument assembly was calibrated as follows. The meter or instrument assembly of this invention including all lead lines was hooked up to this tower and the differential pressure Foxboro cell was zeroed with 3 lbs. p. s. i. g. output transmitted air and no differential across the instrument assembly.- The instrument assembly was installed across the bottom section of the debutanizer tower and across about the lower 10 trays of the 30 tray tower. Then both bottom and top seal legs were sealed with water. Then by means of compressing the spring 108 of the compensator 56, the differential pressure Foxboro cell was again balanced to Zero which was indicated by the 3 lbs. per square inch outlet air pressure. Then by means of an air pressure regulator and a pressure gage connected to the bottom seal pot, like pot 44, on the tower and the top seal pot, like seal pot 32, open to atmosphere, a regulated-air pressure was supplied to the high pressure side of the measuring element (diaphragm of the Foxboro cell) through the bottom seal leg, like leg 50, 52, and compensating unit. It required a difierential pressure of 4.75 p. s. i. g. across the diaphragm to indicate a maximum reading on the pressure gage or recording chart associated with the Foxboro cell. This is known as full range calibration. The pressure of 4.75 p. s. i. g. was applied to the high pressure side with the low pressure side open to atmosphere through the seal leg. The seal legs like legs 33, 50, 52 are then put in operation as above described. Half range required an air pressure of 2.37 p. s. i. g. which illustrates the linearity of the meter.

The instrument assembly of the present invention may be used on any size fractionating tower having any pressure drop because the compensating spring can have its compression adjusted and weaker or stronger springs can be substituted for the spring found to be inadequate.

In one form of the device, the housing 80 is about 8 inches long and about 2 inches in outside diameter and about 1%; inches internal diameter. The plunger 86 is about 4 /2 inches long and has a head 104 of about 1% inches in diameter. The spring 108 is about 1% inches in diameter and about 3 inches long. The pressure connection 122 has a A inch screw tap. Connection 122 has an outside diameter of about 1% inches. The wall thickness of housing 80 is about inch. The depression 106 in plunger head 104 and the depression 116 in adjusting plug 114 are about 1 in. in diameter. The small end 83 of plunger 78 has a diameter of about of an inch and the larger portion 86 of the plunger 78 in passageway 88 is about 3 of an inch with the passageway 88 being about 1 of an inch. The enlarged space 102 has a diameter of about 1% inches. The internal threads extend for about 2% inches.

The above dimensions are given for one form of the device for purposes of illustration only and variations and changes may be made without departing from the spirit of the invention.

What is claimed is:

1. A compensating device of the character described adapted to be removably attached to a'pressure measuring instrument including a hollow housing having at one end areduced externally threaded portion adapted for fluid tight engagement with a pressure measuring instrument, the opposite end of said housing being internally threaded, an adjusting plug threaded into the last-mentioned end of said housing, said reduced housing portion being provided with a small passageway, a plunger having an enlarged head within said housing and a shaft portion which extends through said passageway .and beyond the end of said housing and into said pressure measuring instrument, a coilspring arranged between said enlarged head and said adjusting plug for adjusting compression on said spring, said housing adjacent said spring being provided with a conduit leading from said housing whereby pressure may be supplied to the interior of the housing.

2. A device of the'character described in claim 1 wherein said passageway through which said shaft portion extends is a relatively loose fit to permit passage of fluid therethrough.

3. A combination of the character described including a diaphragm type pressure differential transmitter having a high pressure side and a low pressure side and having its main diaphragm modified to include a socket on the high pressure side, said low pressure side being adapted to be connected with an upper pressure tap of a tower through a seal leg, said high pressure side being adapted to be connected with a lower pressure tap of-a tower through a second seal leg, compensating means for adjusting for the difference in the heads or" liquid in the seal legs, said compensating means being removabiy'attached to said transmitter at the high pressure side and being connected to the seal leg for the lower pressure tap, said compensating means including a housing, plunger means slidably mounted in said housing and extending beyond one end of said housing and held in pressure contact with said socket on said high pressure side of the diaphragm of said transmitter, an adjusting plug threaded into the other end of said housing, and spring means between said plunger means andsaid adjusting plug.

4. In combination with a diaphragm type pressure differential transmitter having a main diaphragm, a vertical seal leg adapted to be connected with the upper portion of a tower, a vertical shorter seal leg adapted to be attached to the lower portion of the same tower, said seal legs terminating at their lower ends at about the same level, said seal legs being adapted to contain liquid and therefore produce different pressures at the lower end of the seal legs, said longer vertical seal leg at its lower end connected to one side of said pressure difiierential transmitter, a compensating device removably connected to the other end of said pressure differential transmitter, the

diaphragm :of said pressure differential transmitter, said plunger having an enlarged head at its inner end within said housing, a spring within said housing and contacting said enlarged head, and means associated with said housing for adjusting the compression on said spring and for maintaining the end of the plunger in engagement with said main diaphragm of said pressure differential transmitter.

5. An apparatus as defined in claim 4 wherein said main diaphragm is provided with a socket to receive the end of said plunger.

6. An apparatus for determining the pressure drop between two levels in a tower which apparatus includes a diaphragm type pressure differential transmitter, a vertical seal leg adapted to be connected with the upper portion of a tower, a vertical shorter seal leg adapted to be attached to'the lower portion or" the same tower, said seal legs terminating at their lower ends at about the same level, said seallegs being adapted to contain liquid and therefore produce different pressures at the lower end of the seal legs, said longer vertical seal leg being connected to one side of said pressurediiferential transmitter, a compensating device removably connected to the other end of said pressure diiferential transmitter, the lower end of said shorter vertical seal leg being connected to said compensating device, said compensating device including a housing, a plunger within said housing and having a reduced'end extending from said housing and held in contact with said main diaphragm of said pressure differential transmitter, said plunger having an enlarged head at its inner end within said housing, a spring within said housing and contactng said enlarged head, an internal adjusting plug threaded into the end of the housing at the end opposite to the end attached to said pressure differential transmitter for adjusting the compression on said spring and for maintaining the end of the plunger in engagement with said main diaphragm of said pressure diiferential transmitter.

7. An apparatus as defined in claim 6 wherein said reduced end of said pluner is slidably mounted in a passageway in said housing which permits passage of fluid to and from said housing and said pressure difierential transmitter.

8. An apparatus as described in claim 6 in which the upper ends of said seal legs are provided with reservoirs larger than the seal legs for containing relatively large amounts of liquid as compared tothe amount of liquid adapted to be contained in said seal legs.

References (Iited in the file of this patent UNITED STATES PATENTS 2,178,927 Campbell Nov. 7, 1939 2,520,547 Hughes Aug. 29, 1950 2,584,455 Hughes Feb. 5, 1952 2,648,345 Markson Aug. 11, 1953 FOREIGN PATENTS 24,426 Great Britain Oct. 27, 1913 

